The invention relates to micro-lithography and, more particularly, to a mask with a focus measurement pattern and a method for measuring a focus value in an exposure process using the same.
When manufacturing an integrated circuit device or a semiconductor device, a micro-lithography process of transferring circuit patterns designed on a mask onto a wafer is performed. The micro-lithography process includes an exposure stage in which circuit patterns on a mask are transferred onto a wafer by using an exposure apparatus, and a stage performing a selective etching using an exposed resist pattern.
At this time, the accurate transfer of the circuit patterns oil the mask onto the wafer depends on an extent of focus change in the exposure stage. Due to various geometrical elements or optical elements, the focus value changes for wafer regions when transferring the patterns onto the wafer. The exposure stage is performed at the optimum focus value which is set through experiments, a priori, or simulations. However, the practical focus value may change from the optimum focus value for the wafer regions, which causes a deformed pattern transfer onto the wafer when compared to the exposure performance at the optimum focus value. In order to control the extent of focus change, a focus measurement is performed. In order to achieve the stable pattern transfer, the extent of focus change applied to the wafer and the focus margin (an allowable extent in the manufacturing process) are measured and checked.
In the process of transferring the circuit patterns of the mask onto the wafer, a focus measurement pattern, which is formed on a region (e.g., a scribe lane region) except for a chip region onto which the circuit patterns are transferred, is used for the focus measurement. When the focus measurement pattern provided on the mask is transferred onto the wafer in the exposure process, the extent of focus change is measured by measuring the extent of deformation occurring to the focus measurement pattern.
The focus measurement pattern is designed such that when the focus value changes, the deformation of the patterns transferred onto the wafer is caused. The change of the focus value happens when the focus of the incident light onto the surface of the wafer changes along a Z-axis direction, perpendicular to the surface of the wafer. Accordingly, the focus measurement pattern is configured such that the focus change along the Z-axis causes a shift of the patterns formed on the wafer. For example, the pattern formed by selectively etching the transparent mask substrate is used as the focus measurement pattern. Such a focus measurement pattern is disposed adjacent to an overlay measurement pattern including a light-shielding pattern of chromium (Cr).
A phase difference between transmitted light is generated between the surface of the mask substrate and the etched phase reversal pattern. Accordingly, the progressing direction of the light changes in a slanted direction at a predetermined angle from the incident direction, i.e., the vertical direction. The focus height changes in the Z-axis direction on the wafer, depending upon the slanted angle of the exposure light, and the transfer position of the measurement pattern on the wafer changes. The extent of focus change is quantitatively measured based on the extent of shift of the measurement pattern transferred onto the wafer in the XY-axis directions.
In the process of forming the focus measurement pattern, when etching the mask substrate for the phase reversal region, it is necessary that the etching is achieved very uniformly. If the uniformity of the etched pattern is relatively low, the extent of pattern shift on the wafer cannot reliably represent the degree of focus change. However, because it is so difficult to uniformly perform the etching on the mask substrate, there is considerable difficulty in the accurate focus measurement. The etched pattern on the mask for the phase reversal region is formed adjacent to the light-shielding pattern of chromium. Thus, an additional selective etching process is required after the formation of the light-shielding pattern of chromium. Also, attack or damage may occur on the light-shielding pattern of chromium or the surface of the mask substrate in the etching process, and it causes quality degradation of the mask.